Traditionally, physical-chemical methods have been used to remove metals from industrial and mining effluents, being the precipitation by rising the pH of the solution the simplest method to perform this operation. However, the permitted levels for metals discharge to surface and underground water bodies are, in some cases, lower to those that can be achieved by this system. For this reason, other additional alternatives for the metals removal have been studied.
In an interesting recent review, Blais et al. (2000). State of the art of technologies for metal removal from industrial effluents. Rev. Sci. Eau 12 (4): 687-711, described in detail the state of development of each of them. Among the metals removal and recovering processes are: precipitation, absorption and biosorption, electro-winning and electrocoagulation, cementation, membrane separation (reverse osmosis and electrodialysis), solvent extraction and ion exchange.
In Table 1 are shown the performance characteristics of these technologies. This information clearly proves that efficiency depends on the tolerance degree of the technologies to the conditions of the effluent and on the required levels of treatment. In this sense, as we will see later, biosorption is a technology that makes possible the removal of metals in order to reach low levels of pollution. Thus, the choice of a suitable pollution control technology depends on several factors, such as:                discharge fate        metal type and its concentration        other contaminants in the water        decontamination level required        treatment costs        
TABLE 1Performance characteristics of some metal removal and recovery technologies(modified from Eccles (1998) Treatment of metal-contaminated wastes: whyselect a biological process? Trends Biotechnol. 17, pp. 462-465.)AppropriatedWorkSuspendedTolerance toLevel forMetalsSolidsOrganicMetalTechnologypH ChangeSelectivityInfluenceMolecules(mg/L)HydroxideTolerantNon-TolerantTolerant>10PrecipitationSelectiveSulfideLimitedLimitedTolerantTolerant>10PrecipitationToleranceSelectivity(according topH)AdsorptionLimitedModeratedGets fouledCan suffer<10tolerancepoisoningElectrochemicalTolerantModeratedCan be handledCan be adjusted>10to becometolerantMicrofiltrationLimitedModeratedGets fouledNon-Tolerant>10Ion exchangeLimitedTheGets fouledCan get<100Tolerancecoordinatingpoisonousresins can beselectiveSolventTolerance inThere areGets fouledNon-Tolerant>100extractionsomeselectivesystemsextractants formetalsBiosorption**LimitedModeratedGets fouledTolerant<10Tolerance**added to comparison
As the environmental restrictions have become more rigorous and the discharge limits of emission have dropped, more environmentally friendly metal production processes are required.